Hodgkin's disease of nodular sclerosis and mixed cellularity subtypes is frequently associated with eosinophilia. To determine if interleukin-5 (IL-5) is implicated in producing the eosinophilia, we performed in situ hybridization studies on cytopreparations of 16 cases of Hodgkin's disease with eosinophilia as well as cells from various controls. A single-stranded, anti-sense complementary DNA (cDNA) probe coding for a portion of the human IL-5 molecule was tail-labeled with digoxigenin -11-dUTP using terminal transferase, and then hybridized to messenger RNA (mRNA) within cells. An alkaline-phosphatase-conjugated antibody directed to digoxigenin was used with a chromogenic substrate to detect hybridized probe within cells. In all seven cases of nodular sclerosis subtype and nine cases of mixed cellularity subtype with eosinophilia, there was strong hybridization signal localizable to the cytoplasm of morphologically identifiable Reed-Sternberg cells and variants. Similar activity was detected only in rare cells from three normal spleens, and was undetectable in two cell lines used as a negative control and in one case of Hodgkin's disease without eosinophilia. Pretreatment of the cytopreparations with the RNase inhibitor diethylpyrocarbonate greatly increased the hybridization signal. Based on this controlled study, we conclude that mRNA coding for IL-5 is expressed in Reed-Sternberg cells and variants. This observation may explain the eosinophilia associated with Hodgkin's disease and provide insight into the origin of the Reed-Sternberg cell.